It's limiting at the moment because it's not finished yet. And Firefox in particular has far outpaced any third-party or industry-wide standards in adding new APIs. They've been proactive and responsive in getting feedback from addon developers while designing the APIs. I would say security addons are one of the top priorities. For example here's a blog post by the NoScript developer: https://blog.mozilla.org/addons/2017/08/01/noscripts-migrati... "I feel that Mozilla has the most flexible and dynamically growing browser extensions platform"